MOBILE COMPUTING. Dynamic Host Configuration Protocol

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1 MOBILE COMPUTING CSE 40814/60814 Fall 2015 Dynamic Host Configuration Protocol Application simplification of installation and maintenance of ed computers supplies systems with all necessary information, such as IP address, DNS server address, domain name, subnet mask, default router etc. enables automatic integration of systems into an Intranet or the Internet, can be used to acquire a COA for Mobile IP Client/Server-Model the client sends via a MAC broadcast a request to the DHCP server (might be via a DHCP relay) DHCPDISCOVER DHCPDISCOVER server client client relay 1

2 time 11/15/15 DHCP Protocol Mechanisms server (not selected) initialization client server (selected) determine the configuration DHCPDISCOVER DHCPDISCOVER determine the configuration DHCPOFFER collection of replies DHCPOFFER selection of configuration DHCPREQUEST (reject) DHCPREQUEST (options) confirmation of configuration DHCPACK initialization completed release DHCPRELEASE delete context DHCP Characteristics Server several servers can be configured for DHCP, coordination not yet standardized (i.e., manual configuration) Renewal of configurations IP addresses have to be requested periodically, simplified protocol Options available for routers, subnet mask, NTP ( time protocol) timeserver, SLP (service location protocol) directory, DNS (domain name system) Big security problems! no authentication of DHCP information specified 2

3 Mobility Home : permanent home of mobile (e.g., /24) Home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent address: address in home, can always be used to reach mobile e.g., wide area correspondent Mobility Permanent address: remains constant (e.g., ) Visited : in which mobile currently resides (e.g., /24) Care-of-address: address in visited. (e.g., ) wide area Correspondent: wants to communicate with mobile 3

4 Finding Somebody Let routing handle it: routers advertise permanent address of mobile-nodesin-residence via usual routing table exchange routing tables indicate where each mobile located no changes to end-systems NOT SCALABLE! Let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile Mobility: Registration home visited 2 wide area foreign agent contacts home agent: this mobile is resident in my 1 mobile contacts foreign agent on entering visited End result: Foreign agent knows about mobile Home agent knows location of mobile 4

5 Mobility via Indirect Routing home correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area 2 foreign agent receives packets, forwards to mobile 4 3 visited mobile replies directly to correspondent Indirect Routing: Comments Mobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparent to correspondent) care-of-address: used by home agent to forward datagrams to mobile foreign agent functions may be done by mobile itself triangle routing: correspondent-home--mobile inefficient when correspondent, mobile are in same 5

6 Indirect Routing: Moving Between Networks Suppose mobile user moves to another registers with new foreign agent new foreign agent registers with home agent home agent updates care-of-address for mobile packets continue to be forwarded to mobile (but with new care-ofaddress) Mobility, changing foreign s transparent: ongoing connections can be maintained! Mobility via Direct Routing correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile visited home 4 correspondent requests, receives foreign address of mobile 2 1 wide area 3 4 mobile replies directly to correspondent 6

7 Mobility via Direct Routing: Comments Overcome triangle routing problem Non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited? Accommodating Mobility with Direct Routing Anchor foreign agent: anchor FA in first visited Data always routed first to anchor FA When mobile moves: new FA arranges to have data forwarded from old FA (chaining) correspondent wide area 1 correspondent agent anchor foreign agent new foreign agent foreign net visited at session start 2 new foreign 7

8 Mobile IP RFC 3220 Has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-ofaddresses, encapsulation (packet-within-a-packet) Three components to standard: indirect routing of datagrams agent discovery registration with home agent Mobile IP: Indirect Routing foreign-agent-to-mobile packet packet sent by home agent to foreign agent: a packet within a packet dest: dest: dest: Permanent address: dest: Care-of address: packet sent by correspondent 8

9 Mobile IP: Agent Discovery Agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) H,F bits: home and/or foreign agent type = 9 code = 0 = 9 router address checksum = 9 standard ICMP fields R bit: registration required type = 16 registration lifetime length sequence # RBHFMGV bits 0 or more care-ofaddresses reserved mobility agent advertisement extension Mobile IP: Registration Example home agent HA: foreign agent COA: ICMP agent adv. COA: visited : /24 Mobile agent MA: registration req. COA: HA: MA: Lifetime: 9999 identification: 714 encapsulation format. registration req. COA: HA: MA: Lifetime: 9999 identification:714. registration reply time HA: MA: Lifetime: 4999 Identification: 714 encapsulation format. registration reply HA: MA: Lifetime: 4999 Identification:

10 Cell Network cell q covers geographical region q base station (BS) analogous to AP q mobile users attach to through BS q air-interface: physical and link layer protocol between mobile and BS MSC q connects cells to wide area net q manages call setup q handles mobility Mobile Switching Center Mobile Switching Center Public telephone, and Internet wired Mobility Management Challenge: roaming message destination Location management Roaming management Handoff management 10

11 Example: Cellular Networks Mobility in Cellular Networks Home : of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) home location register (HLR): database in home containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another ) Visited : in which mobile currently resides visitor location register (VLR): database with entry for each user currently in could be home 11

12 GSM: Indirect Routing home MSC consults HLR, gets roaming number of mobile in visited mobile user HLR 2 4 home home Mobile Switching Center VLR Mobile Switching Center visited 3 correspondent 1 call routed to home Public switched telephone home MSC sets up 2 nd leg of call to MSC in visited MSC in visited completes call through base station to mobile GSM: Handoff with Common MSC old BSS VLR Mobile Switching Center old routing new routing new BSS Handoff goal: route call via new base station (without interruption) Reasons for handoff: stronger signal to/from new BSS (continuing connectivity, less battery drain) load balance: free up channel in current BSS GSM doesn t mandate why to perform handoff (policy), only how (mechanism) Handoff initiated by old BSS 12

13 GSM: Handoff with Common MSC old BSS VLR Mobile Switching Center new BSS 1. old BSS informs MSC of impending handoff, provides list of 1 + new BSSs 2. MSC sets up path (allocates resources) to new BSS 3. new BSS allocates radio channel for use by mobile 4. new BSS signals MSC, old BSS: ready 5. old BSS tells mobile: perform handoff to new BSS 6. mobile, new BSS signal to activate new channel 7. mobile signals via new BSS to MSC: handoff complete. MSC reroutes call 8 MSC-old-BSS resources released GSM: Handoff Between MSCs home Home MSC anchor MSC MSC correspondent PSTN MSC MSC (a) before handoff Anchor MSC: first MSC visited during call call remains routed through anchor MSC new MSCs add on to end of MSC chain as mobile moves to new MSC IS-41 allows optional path minimization step to shorten multi-msc chain 13

14 GSM: Handoff Between MSCs home Home MSC anchor MSC MSC correspondent PSTN MSC MSC Anchor MSC: first MSC visited during call call remains routed through anchor MSC new MSCs add on to end of MSC chain as mobile moves to new MSC IS-41 allows optional path minimization step to shorten multi-msc chain (b) after handoff Impact on Higher Layer Protocols logically, impact should be minimal best effort service model remains unchanged TCP and UDP can (and do) run over wireless, mobile but performance-wise: packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff TCP interprets loss as congestion, will decrease congestion window unnecessarily delay impairments for real-time traffic limited bandwidth of wireless links 14

15 UDP User Datagram Protocol Unreliable and unordered datagram service Adds multiplexing No flow control Endpoints identified by ports servers have well-known ports see /etc/services on Unix Header format SrcPort DstPort Length Checksum Optional checksum Data TCP Transmission Control Protocol Connection-oriented Byte-stream app writes bytes TCP sends segments app reads bytes Full duplex Flow control: keep sender from overrunning receiver Congestion control: keep sender from overrunning Application process Application process Write bytes Read bytes TCP Send buffer TCP Receive buffer Segment Segment Transmit segments Segment 15

16 TCP SrcPort DstPort SequenceNum Acknowledgment HdrLen 0 Flags Checksum AdvertisedWindow UrgPtr Options (variable) Data TCP s 3-way handshake Active participant (client) Passive participant (server) SYN, SequenceNum = x SYN+ACK, SequenceNum=y, Acknowledgment =x+1 ACK, Acknowledgment =y+1 16

17 Connection Termination First participant Second participant FIN, SequenceNum = x ACK, Acknowledgment=x+1, FIN, SequenceNum = y, Acknowledgment = x+1 ACK, Acknowledgment =y+1 Motivation Transport protocols typically designed for fixed end-systems fixed, wired s Research activities performance congestion control efficient retransmissions TCP congestion control packet loss in fixed s typically due to (temporary) overload situations router have to discard packets as soon as the buffers are full TCP recognizes congestion only indirect via missing acknowledgements, retransmissions at full sending rate unwise, they would only contribute to the congestion and make it even worse slow-start algorithm as reaction 17

18 Packet Loss Arriving packet Next free buffer Next to transmit Source 1 Source 2 10-Mbps Ethernet 100-Mbps FDDI Router 1.5-Mbps T1 link Destination (a) Arriving packet Free buffers Queued packets Next to transmit (b) Drop TCP Congestion Control Idea assumes best-effort (FIFO or FQ routers) each source determines capacity for itself uses implicit feedback ACKs pace transmission (self-clocking) Challenge determining the available capacity in the first place adjusting to changes in the available capacity 18

19 AIMD Objective: adjust to changes in the available capacity New state variable per connection: CongestionWindow limits how much data source has in transit MaxWin = MIN(CongestionWindow, AdvertisedWindow) EffWin = MaxWin - (LastByteSent LastByteAcked) Idea: increase CongestionWindow when congestion goes down decrease CongestionWindow when congestion goes up Question: how does the source determine whether or not the is congested? Answer: a timeout occurs timeout signals that a packet was lost packets are seldom lost due to transmission error lost packet implies congestion AIMD Algorithm increment CongestionWindow by one packet per RTT (linear increase) divide CongestionWindow by two whenever a timeout occurs (multiplicative decrease) Source Destination 19

20 AIMD Sawtooth behavior Time (seconds) 10.0 Slow Start Objective: determine the available capacity in the Source beginning Idea: begin with CongestionWindow = 1 packet double CongestionWindow each RTT (increment by 1 packet for each ACK) Destination 20

21 Slow Start Exponential growth, but slower than all at once Used when first starting connection when connection goes dead waiting for timeout Trace Time (seconds) Problem: lose up to half a CongestionWindow s worth of data Fast Retransmit/Fast Recovery TCP sends an acknowledgement only after receiving a packet If a sender receives several acknowledgements for the same packet, this is due to a gap in received packets at the receiver However, the receiver got all packets up to the gap and is actually receiving packets Therefore, packet loss is not due to congestion, continue with current congestion window (do not use slowstart) Packet 1 Packet 2 Packet 3 Packet 4 Packet 5 Packet 6 Retransmit packet 3 Sender Receiver ACK 1 ACK 2 ACK 2 ACK 2 ACK 2 ACK 6 21

22 Fast Retransmit/Fast Recovery Fast recovery skip the slow start phase Time (seconds) go directly to half the last successful CongestionWindow (ssthresh) Mobility Affecting TCP-Mechanisms TCP assumes congestion if packets are dropped typically wrong in wireless s, here we often have packet loss due to transmission errors furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from one access point (e.g., foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible The performance of an unchanged TCP degrades severely however, TCP cannot be changed fundamentally due to the large base of installation in the fixed, TCP for mobility has to remain compatible the basic TCP mechanisms keep the whole Internet together 22

23 Early Approach: Indirect TCP Indirect TCP or I-TCP segments the connection no changes to the TCP protocol for hosts connected to the wired Internet, millions of computers use (variants of) this protocol optimized TCP protocol for mobile hosts splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer hosts in the fixed part of the net do not notice the characteristics of the wireless part mobile host access point (foreign agent) wired Internet wireless TCP standard TCP I-TCP Socket and State Migration access point 1 socket migration and state transfer Internet mobile host access point 2 23

24 Indirect TCP Advantages no changes in the fixed necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work transmission errors on the wireless link do not propagate into the fixed simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host therefore, a very fast retransmission of packets is possible, the short delay on the mobile hop is known Disadvantages loss of end-to-end semantics, an acknowledgement to a sender does now not any longer mean that a receiver really got a packet, foreign agents might crash higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent Snooping TCP Transparent extension of TCP within the foreign agent buffering of packets sent to the mobile host lost packets on the wireless link (both directions!) will be retransmitted immediately by the mobile host or foreign agent, respectively (so called local retransmission) the foreign agent therefore snoops the packet flow and recognizes acknowledgements in both directions, it also filters ACKs changes of TCP only within the foreign agent local retransmission foreign agent wired Internet correspondent host mobile host snooping of ACKs buffering of data end-to-end TCP connection 24

25 Snooping TCP Data transfer to the mobile host FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out fast retransmission possible, transparent for the fixed Data transfer from the mobile host FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH MH can now retransmit data with only a very short delay Advantages end-to-end semantics preserves, no changes to correspondent host (and few to mobile host) no state handover (time-out and retransmission) handover: next FA may not use this approach Problems snooping TCP does not isolate the wireless link as good as I-TCP snooping might be useless depending on encryption schemes Mobile TCP Special handling of lengthy and/or frequent disconnections M-TCP splits as I-TCP does unmodified TCP fixed to supervisory host (SH) optimized TCP SH to MH Supervisory host no caching, no retransmission monitors all packets, if disconnection detected set sender window size to 0 sender automatically goes into persistent mode old or new SH reopen the window Advantages maintains semantics, supports disconnection, no buffer forwarding, no changes to sender s TCP Disadvantages loss on wireless link propagated into fixed adapted TCP on wireless link 25

26 Fast Retransmit/Fast Recovery Change of foreign agent often results in packet loss TCP reacts with slow-start although there is no congestion Forced fast retransmit as soon as the mobile host has registered with a new foreign agent, the MH sends duplicated acknowledgements on purpose this forces the fast retransmit mode at the communication partners additionally, the TCP on the MH is forced to continue sending with the actual window size and not to go into slow-start after registration Advantage simple changes result in significant higher performance Disadvantage focus on handover losses (not wireless link errors) Transmission/Time-out Freezing Mobile hosts can be disconnected for a longer time no packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells with higher priority traffic TCP disconnects after time-out completely TCP freezing MAC layer is often able to detect interruption in advance MAC can inform TCP layer of upcoming loss of connection TCP stops sending, but does not assume a congested link MAC layer signals again if reconnected Advantage scheme is independent of data Disadvantage TCP on mobile host has to be changed, mechanism depends on MAC layer 26

27 Selective Retransmission TCP acknowledgements are often cumulative ACK n acknowledges correct and in-sequence receipt of packets up to n if single packets are missing quite often a whole packet sequence beginning at the gap has to be retransmitted (go-back-n), thus wasting bandwidth Selective retransmission as one solution RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps sender can now retransmit only the missing packets Advantage much higher efficiency Disadvantage more complex software in a receiver, more buffer needed at the receiver Transaction Oriented TCP (T/TCP) TCP phases connection setup, data transmission, connection release using 3-way-handshake needs 3 packets for setup and release, respectively thus, even short messages need a minimum of 7 packets! Transaction oriented TCP RFC1644, T/TCP, describes a TCP version to avoid this overhead connection setup, data transfer and connection release can be combined thus, only 2 or 3 packets are needed Advantage efficiency Disadvantage requires changed TCP mobility not longer transparent 27

28 Comparison of Different Approaches for a Mobile TCP Approach Mechanism Advantages Disadvantages Indirect TCP splits TCP connection into two connections isolation of wireless link, simple loss of TCP semantics, higher latency at handover Snooping TCP M-TCP snoops data and transparent for end-toend acknowledgements, local connection, MAC retransmission integration possible splits TCP connection, chokes sender via window size Fast retransmit/ avoids slow-start after fast recovery roaming Transmission/ freezes TCP state at time-out freezing disconnect, resumes after reconnection Selective retransmission Transaction oriented TCP Maintains end-to-end semantics, handles long term and frequent disconnections simple and efficient independent of content or encryption, works for longer interrupts problematic with encryption, bad isolation of wireless link Bad isolation of wireless link, processing overhead due to bandwidth management mixed layers, not transparent changes in TCP required, MAC dependant retransmit only lost data very efficient slightly more complex receiver software, more buffer needed combine connection setup/release and data transmission Efficient for certain applications changes in TCP required, not transparent 28